In order to meet the control requirements of the surface accuracy and structural stability of large aperture mirrors for laser devices, a multi-point mounting method with decoupling of multiple degrees of freedom of the mirror is proposed. The control of multiple degrees of freedom of mirror is realized by limiting position to avoid the additional surface shape caused by mounting. The effectiveness of the proposed method has been analyzed by finite element method, and the feasibility of the analysis method and the mounting method has been verified through experiments. The results show that the additional surface shape brought by the mounting method of the mirror is small, which meets the requirement of the low-stress additional mounting surface of the mirror. On the basis, the surface shape of the mirror placed at a tilt angle of 45° is simulated, and the influences of the position distributions of different mounting points on the surface accuracy of the mirror are explored. Simulation results show that in order to ensure the surface accuracy of the mirror, at least one mounting point should be located on the longer side of the mirror. These results have important guiding significance for the mounting design of large aperture mirrors.